The invention relates to a method of heating the intake air, especially for an internal combustion engine in the starting phase and warm-running phase.
Heating the intake air in internal combustion engines, in particular in diesel internal combustion engines in the starting and warm-running phase, is required for a number of reasons. At low ambient temperatures and consequently low intake-air temperatures, especially in the case of diesel engines, an inadequate final compression temperature occurs, and therefore an increasing emission delay, that is to say the time from the entry of the fuel into the combustion chamber until the ignition of the same becomes too long. Furthermore, at low intake temperatures, local over-enrichment, incomplete combustion and high pressure gradients occur as a result of abrupt mixture conversion in the cylinder. The consequences are a sharply increased emission of hydrocarbons in the exhaust gas and the knocking of the diesel engine, as well as the disadvantages resulting from this, such as severe loading of the environment and increased loading of the parts of the propulsion unit.
The general prior art discloses a method of preheating the intake air in which electrically heatable heating elements are used, and the pre-start glow time and post-start glow time of the same are defined by a time-controlled relay to only one value in each case for the pre-start glow time and the post-start glow time. The pre-start glow time is to be understood as that period during which the heating element is heated and gives off heat to the intake air before the starter sets the crankshaft moving in the known way. The post-start glow time is the period during which the heated heating element gives off heat to the intake air after the starter has begun to rotate the crankshaft of the internal combustion engine. During the pre-start glow time and the post-start glow time, the cold intake air coming from the environment or from a charging-air cooler is heated by flowing past a heating wire of the heating element.
Optimum adaptation of the pre-start glow time and of the post-start glow time of the heating element to the ambient temperature or intake-air temperature is not possible. Thus, on the one hand the pre-start glow time and the post-start glow time can be too long, which means virtually that more electrical energy is consumed than is actually necessary and therefore leads to unnecessary weakening of the capacity of the starter battery. On the other hand, the pre-start glow time and the post-start glow time of the heating elements may be too short, because of extremely low air temperatures, which once more leads to the consequences already mentioned above, such as ignition delay and increasing the HC emissions in the exhaust gas, or to its being impossible for the engine to start up at all. If the pre-start glow time and the post-start glow time of the heating element are to be adapted to the given boundary conditions, an additional control or regulating unit is necessary.
The journal MTZ (Motortechnische Zeitschrift [Motor Engineering Journal] 58, 1997; Issue 4) discloses a method of heating the intake air in which a heating element having a self-regulating heating wire is used. The use of a self-regulating heating element is intended to adapt the heating of the intake air in an optimum way to the given air temperatures.
The disadvantage with this method is that a heating flange which is more complicated in its construction and therefore more expensive is required to implement it. Furthermore, this method cannot be extended to diesel internal combustion engines which are already in use or to internal combustion engines which are operated with relatively frequently changing fuels having a very different ignition or combustion behaviour, without a considerable outlay on conversion being necessary for this. In addition, it is not possible for the heating element used in this method to react to other boundary conditions than the intake-air temperature.